1. Field of the Invention
The present invention relates to a group III-nitride light emitting device. More particularly, the present invention relates to a group III-nitride light emitting device such as a group III-nitride light emitting diode (LED) or a laser diode improved in operating voltage and electrostatic discharge characteristics.
2. Description of the Related Art
A device using a group III-nitride semiconductor is utilized in many areas such as light emitting devices including light emitting diodes (LEDs) and laser diodes (LDs) or light-receiving devices such as solar cell and optical censor. The device is also employed in electronic devices such as a transistor and a power device. The group III-nitride semiconductor is a compound semiconductor having a composition expressed by AlxGayIn1-x-yN, where 0≦x≦1, 0≦y≦1 and 0≦x+y≦1. The LED and LD devices using such material are essential for the development of various light sources such as a natural-color sign board, a traffic signal, a light source of image scanner, and a high-density optical recording medium.
FIG. 1 is a sectional view illustrating a group III-nitride light emitting device (especially LED device). Referring to FIG. 1, a conventional light emitting device 10 includes an undoped GaN layer 13, an n-type clad layer 14 made of n-doped AlxGayIn1-x-yN, an active layer 16 and a p-type clad layer 18 made of p-doped AlxGayIn1-x-yN. An n-electrode 24 is formed on a surface of the n-type clad layer 14 exposed by mesa etching, and a transparent electrode layer 20 and a p-type electrode 22 made of ITO are formed on the p-type GaN-based clad layer 18. Japanese Laid-Open Patent Application No. 10-135514 discloses an active layer of a multiple quantum well structure that has an undoped GaN barrier layer and an undoped InGaN well layer in order to enhance light emitting efficiency and brightness.
However, in order to use the LED device 10 described above as lighting source and outdoor display, output power should be enhanced more and operating voltage (Vf) should be lowered further to reduce heat generation and improve reliability and useful life of the device. Also, to utilize the LD device as stable light source for optical pickup or DVD, it is necessary to upgrade the LD device significantly and lengthen its useful life through reduction in operating voltage.
In addition to operating voltage problem as identified above, electrostatic discharge (ESD) is caused by a person or a machine handling or using the LED or LD. Extensive studies have been conducted to increase ESD resistance in order to prevent damage to the LED or LE by ESD, and various solutions have been proposed. For example, optical device such as the LED or LD can be connected in parallel to a separate zener diode, or an ESD protection diode is mounted on a single substrate in manufacturing of the LED (refer to U.S. Pat. No. 6,593,597). But these methods require purchasing or assembling of a separate protective device besides the LED (or LD) or providing a protective LED via a separate process, therefore complicating a manufacturing process and increasing costs.